This Webpage has been added
to my website at the request of many Radio Amateurs, who have
listened to my transmissions on TOP BAND. I
am told that my signal doesn't sound like the original Chatterbox
design and that they would like to know what my Modifications
are, so that they can incorporate them into their own version.Unfortunately, this is
not an easy task to do, as some of the modifications made were taken from
a number of unknown sources. Some of the modifications were suggested
by my students who may have received their information from copyright material.I have therefore not reproduced
the circuit diagrams here, so as not to breach any copyright.(Although I feel that the Authors would give
their permission if asked, unfortunately they are not all known to me and
so I have made this decision)I have however, given details
of my Low-Pass Filter, which if not fitted to the output of the Transmitter,
would have allowed a large number of harmonics to be radiated, giving possible
rise to interference - also, a BFO for the reception of SSB
and AM on the Receiver. The BFO is very stable, and I use it when
SSB or CW stations call in to make contact.

I wish to point out that the
Chatterbox Transmitter and Receiver was originally designed by G3RJV (The
Rev. George Dobbs) and G3ROO (Ian Keyser). But for their efforts,
I would not have started the project. The information given below
is only given as a guideline.
If anyone would
like a copy of the original article, then they should either contact the
authors or write to Practical
Wireless. (Printed Circuit Boards for this Project can
be obtained from the PW PCB Service). The original articles appeared in
Practical Wireless Magazine Aug/Sept 1991.

I became personally involved
with the Chatterbox Transmitter and Receiver in October 1993. My
Construction class for the Radio Amateur's Course had just started at the
Local College and I was looking for a project - one to get the students
involved in building their own Transmitters and Receivers.To cut a long story short,
there were many problems with the Transmitter. I decided that the only
way to resolve the problems quickly, was for me to build a prototype at
home, where I could work without interruptions. The result was very time
consuming, but very rewarding.

The Chatterbox Transmitter

The major modifications were made to the main Transmitter Board.The Transmitter as a whole was unstable. This
was corrected by re-designing the layout of the tracks on the printed
circuit board, followed by fitting additional decoupling capacitors.The Modulation was very poor and distorted.
I have been told by other Radio Amateurs, that it would have been better
to use a larger Audio Power output Integrated Circuit. This would have
required a larger supply voltage - the original design uses +12Volts. The audio level was improved by applying modulation
to both the Driver and the Power Amplifier stages. Many Radio Amateurs have told me that they
have tried commercial modulation transformers with success. I am still
using a home wound Modulation Transformer which has given no problems at
all.

Note: I am aware that a Pye Westminster ex-PMR
Model W15 A.M Modulation Transformer is very similar in size to the 'home
wound version' and fits on the Printed Circuit Board without any problems.
(Part No. AL21219).

A very important
addition to the output stage is that of a Low Pass Filter. Having built
a Spectrum Analyser, I discovered strong harmonics on eighty and forty
metres. The LPF above has a 'Cut-off'' at 2.5MHz.Tests now show
that harmful harmonics (those which cause interference) are completely
eliminated.

Output from Low-Pass Filter using home built Spectrum Analyser.

The circuit above shows the switching arrangements.
Position 1. is to NET. Position 2. is to Receive. Position 3. is OFF. (It
also provides a PAUSE between Receive and Transmit (Position 4.). This
allows time for the Receiver to go dead before the Transmitter activates.
(Plop in the Loudspeaker)

The Speech Processor used with my Chatterbox

Even after boosting
the audio in order to increase the level of modulation, the modulated output
signal was sub-standard. This has been overcome by using a Home Built
Speech Processor. The Speech Processor
was designed around six BC108 Transistors. The measured frequency response
of the completed unit at the -3dB points is 350Hz to 3KHz. The power is
taken from the 12Volt supply used by the Chatterbox. The Speech Processor
requires a 9Volts d.c. supply, and this is provided by the regulated power
supply shown below.

When the Speech Processor
is used, R12 on the Main Transmitter Board is set to the '1/3rd on'
position, and the modulation contains all the extraneous sounds of the
children in the living room. In fact, I have to turn the audio gain down,
as the Transmitter output could easily become over modulated.

The Chatterbox Receiver

The Chatterbox Receiver
is excellent. The only problem being that it doesn't have a BFO fitted.
The Circuit below is that of a Beat Frequency Oscillator which is stable
enough for the reception of Single Sideband. (The Transformer was taken
out of a scrap Transistor Radio Receiver.)

It is worth noting
that:

The Intermediate Frequency is 455 KHz.

The Oscillator Frequency is 1345 - 1545 KHz

The Receiver Tuning Frequency is 1800 - 2000 KHz

The output from this
BFO is quite high. So as not to overload the Receiver it is
necessary to limit the coupling. This is achieved by
connecting a 2pF capacitor to TR1 Collector, and then connecting
the other end to a short length of wire supported about
1cm above IC4 on the Receiver PCB using insulated wire supports.

When building Transmitters,
especially when FETS are used in Power Amplifier Stages, it is important
to check for Harmonics. FETS are prone to generating Harmonics
The best way to check, is to use a Spectrum Analyser.

The Power Supply is 13.8 Volts and is also Home Built. This
is described elsewhere on this Website.
The Maximum Measured Power Output from the G3NGD version is 6.25Watts
[7.9dBW]Currently,
September, 2010, G3NGD is using his New Six-Band Trapped Dipole (14MHz
- 52MHz) with the feeders strapped, Series Tuned against Ground as the
Aerial. This seems to work well and makes the whole set-up a 'Stand Alone'
Home Constructed Station.

The set-up could be used as a portable station and a Kite could be used
as the Aerial.The Aerial
could be tuned through the Tuning Unit already used by G3NGD. This is basically,
29 Turns of 20SWG Copper Wire wound on an 8cm dia. former, tuned
in series with a 2x500pF tuning capacitor (1000pF) taken from an
Old Valve Radio Receiver of fifty years ago. John built his own Box Kites
in the 'Sixties' and transmitted to Ireland and the I.O.M. during the daytime.He operated
Portable from the Beach at Fleetwood.The advantage
of using this set-up is the fact that it will work off the 12V car battery.
In the 'Sixties', the equipment needed 200V D.C. and G3NGD built an Inverter,
12V in and 200V out.The unit used OC35 Power Transistors and the equipment
is shown elsewhere in this website.